UNIT 1 MATTER AND ENERGY<br > MATTER<br > SUBSTANCES. The term subslance is used to refer to any particular<br > variety of matter that always has the same properties and composition，<br > regardless of how and where a specimen of it is obtained. For ex-<br > ample， water is a substanee; under given conditions， any sample of<br > water has the same properties and the same composition. These<br > properties and composition can be used to identify water. On the<br > other hand. wood is not a substance; its properties and composition<br > can vary widely.<br > A sample of any substance is homogeneous; that is， the properties<br > and composition are the same throughout the sample.<br > There are two major kinds of substances:<br > 1. Elements. A substance that cannot be decomposed into two or<br > more other substances by means of a chemical change is called an el-<br > ement. An element consists entirely of atoms with the same atomic<br > number. One hundred and six different elements are known to exist.<br > Most of the elements are metals; typical examples are aluminum，<br >iron copper， silver， and mercury. Some familiar nonmetallic elements<br >are carbon， snlful， iodine， oxygen， and neon.<br > 2. Compounds. A substance that can be decomposed into other<br >substances by a chemical change is called a compound. Every com-<br >pound consists of two or more elements chemically combined in<br >definite proportions by weight. For example， the compound water<br >always consists of the two elements hydrogen and oxygen in a weight<br >ratio of 1:8. That is， if 9 grams of water is decomposed， 1 gram of<br >hydrogen and 8 grams of oxygen are always obtained， The properties<br >of compounds are usually quite different from those of their com-<br >ponent elements.<br >MIXTURES. Like compounds， mixtures are always composed of two<br >or more substances， but mixtures differ from compounds in the fol-<br >lowing ways:<br > l. The components of a mixture can he either elements or com-<br >pounds.<br > 2. The weight ratios of components in a mixture are not fixed.<br > 3， The properties of the mixture are always intermediate between<br >those of its components，<br > 4. Some mixtures， such as solutions of salt in water or mixtures of<br >gases， are homogeneous， but others， such as concrete (a mixture of<br >sand， water， and cement)， are helerogeneous (that is， samples are<br >not uniform in composition throughout).<br > I<br >